Foamed plastics exhibit reduced apparent densities due to the presence of numerous cells dispersed throughout the mass of the polymer. Rigid foams that exhibit densities greater than about 320 kg/m3 are typically referred to as structural foams, and are well known in the art. Structural foams are commonly used in various aspects of manufacturing molded articles in which low density polymeric materials are desirable.
Cellular polymers and plastics are made by a variety of methods typically involving steps of cell initiation, cell growth and cell stabilization. Structural foams having an integral skin, cellular core, and a high strength to weight ratio, are made by several processes, including injection molding and extrusion molding. The particular process that is used is selected based upon requirements of the end products.
Because of the favorable combination of properties, price, and ease of processing, styrene polymers, and especially polystyrene, are widely used in preparing foam sheets, films and slabs for such divergent end uses as packaging, pipe and tubing, construction and insulation. For example, expanded styrene polymers such as polystyrene are widely used in the insulation of freezers, coolers, truck bodies, railroad cars, farm buildings, roof decks and residential housing. Styrene polymer foams are also used as a core material for structural sandwich panels used in refrigerated truck bodies, mobile homes and recreational vehicles. Extruded polystyrene foams (XPS) are widely used to insulate buildings and components of buildings. Extruded polystyrene foams are also used in various food packaging applications.
Initiation of cell formation and promotion of cells of a given size are controlled by nucleation agents included in the polymer composition. The nature of cell control agents added to polymer compositions influences the mechanical stability of the foamed structure by changing the physical properties of the plastic phase and by creating discontinuities in the plastic phase. These discontinuities allow blowing agent(s) used in cell formation to diffuse from the cells to the surrounding material. Typically, the resulting cells provide for a lightweight molded article, but do so at the expense of impact resistance. For example, nucleation agents often promote crystalline structures within the cooled polymer, which reduce impact resistance. Mineral fillers may be added to provide a large number of nucleation sites, but such fillers tend to serve as stress concentrators, thereby promoting crack formation and decreasing the impact resistance of the resulting molded articles. Typically, the reduced strength of structural foams may be at least partially offset by increasing the wall thickness of molded articles. However, increasing wall thickness requires greater amounts of raw materials per unit molded, thereby increasing the cost of production.
Flame retardant (FR) additives are commonly added to extruded polymer foam products that are used in construction and automotive applications. The presence of the flame retardant additive allows the foam to pass standard fire tests as are required in various jurisdictions. Various brominated compounds having low molecular weights, typically less than about 1000 g/mol, are used as flame retardant additives in many of these foam products. Many of these compounds, such as hexabromocyclododecane, are under regulatory and public pressures that may lead to restrictions on their use, and so a strong incentive exists to find a replacement for such brominated compounds.
An alternative flame retardant additive for extruded polymer foams should be capable of allowing the foam to pass standard fire tests when incorporated into the foam at reasonably low levels. Because extruded foams are processed at elevated temperatures, it is important that the flame retardant additive be thermally stable at the temperature conditions used in the extrusion process. For some foams such as polystyrene and styrene copolymer foams, these temperatures are typically 180° C. or higher. Several problems are encountered if the flame retardant additive decomposes during the extrusion process. These problems include loss of flame retardant agent and therefore loss of flame retardant properties, and the generation of decomposition products (such as HBr) that are often corrosive and therefore potentially dangerous to humans and harmful to operating equipment. Therefore, the flame retardant agent should not cause a significant loss of desirable physical properties in the polymer. It is also preferable that the flame retardant additive has low toxicity and is not highly bioavailable.
The incorporation of flame retardant additives into thermoplastic polymer compositions can also negatively impact the strength of the resulting foam product. For example, in foamed styrene polymers containing halogenated additives, the degree of bromine loading must be relatively low to avoid detrimentally impacting the structural qualities and skin quality of the foam. For example, when utilizing hexabromocyclododecane (HBCD) as a fire retardant in a styrene polymer foam, a high level of HBCD is required in order to meet fire retardancy requirements, particularly the stringent European fire retardancy tests. The incorporation of HBCD into the styrene polymer foam at these levels may result in poor skin quality and a high degree of degradation of the styrene polymer and of the reground material in an extrusion process due to excessive heating. Excessive heat and degradation bring about a reduction of the molecular weight of the styrene polymer foam and of the reground styrene polymer and a resultant drop in physical and mechanical properties.
Therefore, a need exists in the art for fire retardant styrene polymer foams which utilize halogenated fire retardants and particularly hexabromocyclododecane (HBCD) as the fire retardant, which meet fire retardancy requirements, and which are amenable to extrusion processes, but which do not exhibit poor structural qualities and/or skin qualities.